Previously, it has been proposed to use gel chromatography to separate platinum group metals (PGMs) from one another on an industrial scale. Prior proposals include U.S. Pat. No. 4,885,143 (Schmuckler). This patent describes a method in which the interseparation of platinum group metals (PGMs) from an oxidised gold-free halide solution is achieved using a chromatographic medium such as a polysaccharide gel (Sephadex) or a polyacrylamide gel (Biogel). The PGMs when dissolved in a chloride solution are absorbed onto the chromatographic column and are claimed to be selectively eluted in the order ruthenium, rhodium, palladium, platinum, iridium and caesium, although it is clear from the rest of the patent that Schmuckler meant osmium rather than caesium. The problem with this method is that there is in fact no clear separation of PGMs.
This problem was to a large extent overcome by European patent application EP 756013 (Matthey Rustenburg Refiners Pty) which describes a method for the interseparation of PGMs from a PGM-containing halide solution comprising the steps of passing the solution through a glycol methacrylate chromatographic medium, absorbing the PGMs onto the medium, eluting each PGM using an acid solution to obtain each fraction containing at least one PGM. During the elution of a reduced, mixed rhodium, iridium, ruthenium, palladium, platinum and osmium/6 molar hydrochloric acid (6 M HCl) solution through Toyopearl HW-40C using a (usually 6 M) hydrochloric acid eluent, the first eluted band contains trivalent rhodium iridium and ruthenium (which will be subsequently referred to as rhodium (III), iridium (III) and ruthenium (III)), that is, the method does not separate rhodium, iridium and ruthenium either from each other or any combinations/permutations within.
The present invention sets out to address the problem of the separation of the metals rhodium, iridium and ruthenium, from one another in addition to other PGMs using gel chromatography. This may be of particular importance, allowing the separation of these metals by chromatography on an industrial scale. Presently, refining processes involve solvent extraction, distillation and ion exchange. The metals are processed sequentially, for example in the order osmium, gold, palladium, platinum, ruthenium, iridium and rhodium. The method of the present invention has several advantages over the previously described methods in allowing simultaneous separation of some or all of these metals. In addition, the process is extremely fast, and the purity and yield of the extracted metals is high.
Ruthenium is unique amongst the platinum group metals in that it forms extremely stable nitrosyl complexes (containing the NO+ moiety) and indeed has a well documented nitrosyl literature (Coord.Chem.rev (1978) 26 (1), 7–32, Mercer et al. Inorganic Chemistry, Vol 3 No7, 1964 pg 1018). An extensive study has been previously performed by the present inventors looking at the potential use of ruthenium nitrosyl species in precious metal refining. Of relevance in the present context is that in 6 M HCl, there are two major ruthenium nitrosyl species present: [Ru(NO)Cl5]2− and [Ru(NO)Cl4(H2O)]−. In both complexes, the ruthenium is formally present in its divalent oxidation state. The [Ru(NO)Cl5]2− and [Ru(NO)Cl4(H2O)]− species are in equilibrium, the equilibrium ratio at 6 M chloride concentration being approximately 2:1. As the chloride concentration decreases, the relative amount of the [Ru(NO)Cl4(H2O)]— species increases and as the chloride concentration increases, the relative amount of the [Ru(NO)Cl5]2− species increases.
There are a number of literature methods (GB 2293372, Matthey Rustenburg Refiners PTY, Spec Publ. -Roy Soc. Chem (1993), 122) for preparing ruthenium nitrosyl species including the use of sodium nitrite, nitric oxide gas and nitric acid with a reductant. The method used to date has involved the use of formic (HCOOH) and nitric (HNO3) acids.